The separation and removal of one gas from a mixture of gases is an important process with many applications. One particular area of concern is the removal of condensible volatile organic compounds (VOCs) from air streams, nitrogen streams and other gas streams in industrial and commercial processes and facilities. For example, VOCs used as carrier liquids and dissolving agents in many industrial processes are vaporized and escape routinely into the atmosphere via process exhaust air streams. The VOCs in such air streams are a serious environmental problem. If the air, N.sub.2, or gas stream is to be safely released, the VOCs must first be removed from such discharges to control environmental pollution. VOCs are precursors to ground level ozone, a major component in the formation of smog. Another example is the removal of CO.sub.2 from air, N.sub.2, methane, or other gases or gas mixtures. A third example deals with the removal of olefin from paraffin. A fourth example is concerned with removal of O.sub.2 from N.sub.2. A fifth example considers removal of H.sub.2 O from a gas stream.
High purification of a gas mixture is generally achieved in industry by means of pressure swing adsorption (PSA) processes. For example, see Gas Separation by Adsorption Processes, Yang, R. T., Butterworths, Boston, 1987. Typically, a gas mixture flows along a bed of adsorbents for a short period of time; the front end of the gas mixture is highly purified and is taken out as the product. After the short period, the flow of the fresh gaseous feed into the bed is stopped to prevent a breakthrough of the feed gas through the product end. The exhausted bed is either evacuated and/or cleaned by a purge gas to regenerate the bed of adsorbents for another cycle of adsorption-based purification of the feed gas mixture. The multi-step process of bed regeneration is generally complex, and PSA beds are bulky.
It would be useful if a membrane device could be economically used to purify the feed gas mixture to the same extent, as membrane devices are compact and modular, and capital costs associated with membranes are generally lower than other devices and processes. However, membrane separation processes which are operated in a conventionally steady-state fashion are known to be efficient only for bulk gas separation. See Integration of Membranes with Other Air Separation Technologies, Beaver, E. R., Bhat P. V., Sarcia D. S., AIChE Symp.Ser., 1988, No.261, vol. 84, 118.
Existing cyclically-operated membrane-based separation processes may be classified as being of two types: high/low pressure swing and adsorbent particle-based/absorbent liquid-based processes.
In the first type of process, polymeric gas separation membrane-based devices are operated with a cyclic pulsing of the gas pressure on the upstream side of the membrane between a high value of P, the feed gas pressure, and p, the permeate side pressure (&lt;P). The permeate side pressure p is always maintained at a low value. For example, see Membrane Separation of Gases Using Steady Cyclic Operation, Paul, D. R., I&E.C. Proc.Des.Dev., 1971, 10, 375. For a gas mixture, such an operation allows an improved selectivity between a rapidly-diffusing "species 1" and a slower-diffusing "species 2". The first fraction of the permeate collected is more enriched in species 1 than is possible in steady-state processes.
A recent minor variation of such a process introduces an inert purge gas or an inert liquid into the feed gas side during the interval when the high pressure feed gas flow on the feed side is stopped. See U.S. Pat. No. 5,354,474 issued to LaPack et al. on Oct. 11, 1994.
Another variation of the process of Paul referenced hereinabove was suggested by Lapack and Dupuis in U.S. Pat. No. 5,354,474 which involves collecting a second permeate fraction more enriched in species 2 during a brief period after the first period used to collect a permeate fraction enriched in species 1.
Yet another variation of this first type of process is suggested by Ueda et al. in U.S. Pat. No. 4,955,998 issued on Sep. 11, 1990 involving the implementation of alternate introduction of feed gas under pressure to the feed side and evacuation of the permeating gas under vacuum to maximize the driving pressure difference between the feed and the permeate side.
It should be noted that steady-state processes often have special provisions for start-up time-dependency. For example, in air dehydration processes by a membrane unit, at steady-state, the membrane unit delivers adequately dehumidified air. However, during start-up with a compressor, there are problems due to low pressure and residual moisture in the permeator from an earlier period. In order to address these problems, a purge gas stream may be introduced at atmospheric pressure from the high pressure dried product gas end during the interrupted period when no high pressure feed gas is being supplied. Usually the purge gas is obtained from the purified high pressure product gas which was obtained from its earlier operation, as described, for example, in U.S. Pat. No. 5,030,251 issued on Jul. 9, 1991 to Rice and Brown. The utility of the high pressure purge stream has also been demonstrated in the production of N.sub.2. In order to avoid using a purge stream in such air dehydration processes as described in U.S. Pat. No. 5,030,251, Brockman and Rice, in U.S. Pat. No. 5,131,929 issued on Jul. 21, 1992, have suggested a delay at the beginning of the process as well as additional condensation of moisture beyond the compressor prior to introduction of the high pressure feed air into the membrane dehumidifier.
In the second type of process, microporous hollow fiber membranes have been employed along with fine adsorbent particles, as described, for example, in Gas Separations in Hollow-Fiber Adsorbers, Gilleskie et al., AIChEJ, 1995, 41, 1413. Alternatively, an aqueous absorbent liquid that does not wet the pores of the membrane has been employed on the shell side, as described, for example, in Hollow-Fiber Membrane Based Rapid Pressure Swing Absorption, Bhaumik, S., Majumdar, S., and Sirkar, K. K., AIChEJ, 1996, 42, 409. Thus, the feed gas mixture flows through the bores of the hollow fine fibers. The adsorbent particle adsorbs specific species from the feed gas mixture; alternately, the aqueous absorbent liquid absorbs the selected species. The high pressure feed gas leaving the exit end of the feed fibers is highly purified, and the feed gas flow is stopped after sometime to prevent a breakthrough of the species at the exit end. The bed of adsorbents or the shell-side liquid absorbent is then regenerated by a variety of demanding multistep procedures borrowed from PSA processes. While such processes are inherently capable of producing a highly purified gas stream from the incoming high pressure feed gas, the bed regeneration process leaves much to be desired especially when rapid cyclic processes are implemented for high levels of feed gas purification. The membrane in such a process does not perform any chemical separation as such.
The citation of any reference herein should not be construed as an admission that such reference is available as prior art to the invention.